In-line screw type injection molding machine and method of controlling the same

ABSTRACT

The invention is to provide an in-line screw type injection molding machine which can have a simplified configuration of an injection mechanism and a metering mechanism. The in-line screw type injection molding machine is characterized by comprising: a metering motor  9  which rotationally drives a rear end side of a screw; a screw mechanism which includes a nut body rotating integrally with the screw, and a screw shaft fitted to the nut body, the screw mechanism converting rotational motion of the screw shaft into linear motion of the screw through the nut body; an injection motor  16  which rotationally drives the screw shaft; an injection motor drive circuit which drives the injection motor  16  to allow a position of the screw to follow a predetermined position command pattern; a metering motor drive circuit which drives the metering motor  9  to allow rotation of the metering motor  9  to follow a predetermined speed setting pattern; and an adder-subtracter circuit which adds or subtracts a speed setting pattern signal of the metering motor drive circuit to or from a speed command signal of the injection motor drive circuit so as to compensate an axial displacement of the screw caused by the rotation of the metering motor  9.

TECHNICAL FIELD

The present invention relates to an in-line screw type injection moldingmachine, and particularly relates to an in-line screw type injectionmolding machine provided with a drive mechanism for rotationally drivinga screw and further axially driving the screw during metering andinjection.

BACKGROUND ART

As an injection molding machine, there is known an in-line screw typeinjection molding machine, which moves a screw backward while rotatingthe screw to melt a plastic material while metering the plasticmaterial, and then suspends the rotation of the screw and moves thescrew forward to inject the molten resin into a mold and fill the moldwith the molten resin. This type injection molding machine has ametering motor as means for rotationally driving the screw, and aninjection motor as means for injecting the molten resin to fill the moldtherewith, and these motors are disposed axially in series (see PatentDocument 1).

The metering motor is, for example, coupled with a base coupled with arear end of the screw, and the injection motor rotationally drives ascrew shaft abutting against the base so as to axially move the basealong the screw shaft.

Then, the metering motor is driven to rotate the screw, while theinjection motor is driven to drive the screw shaft, so as to move thescrew backward while rotating the screw to melt the resin while meteringthe resin and to rotate the screw shaft reversely to thrust the screwforward through the screw shaft to thereby inject the resin.

FIG. 4 is a diagram for explaining a control circuit of an in-line screwtype injection molding machine in the background art. The injectionmolding machine has a screw 6 which is, for example, disposed in aheating cylinder rotatably and movably forward/backward, a meteringmotor 9 which rotationally drives the screw 6, and an injection motor 16which drives the screw 6 forward/backward.

When the screw 6 in the heating cylinder is rotated backward (clockwisein view from the illustrated motor shaft end), raw resin (for example,thermoplastic resin) supplied from a hopper is kneaded in the heatingcylinder and moved toward the front end (left side) of the screw whilebeing plasticized. Thus, metered molten resin is accumulated on thefront end side. Next, the injection motor 16 is rotated forwardsuddenly. On this occasion, the screw 6 is pressed to the illustratedleft so as to move forward suddenly. In this manner, the molten resin isinjected into a not-shown mold through a nozzle.

An injection motor encoder 57 measures the rotational position of theinjection motor 16, and a metering motor encoder 58 measures therotational position of the metering motor 9. The injection encoder 57 isan absolute type encoder which outputs the absolute value of therotational position, and the metering motor encoder 58 is an incrementalencoder. A load cell 49 is a sensor for measuring an injection pressureand a back pressure imposed on the screw 6.

In FIG. 4, xij0 designates a backward position command pattern signalindicating a backward position of the screw, vij0 designates a backwardspeed command pattern signal indicating a backward speed of the screw,bp0 designates aback pressure setting pattern signal for setting a backpressure to be imposed on the screw, and vcg0 designates a meteringmotor rotational speed setting pattern signal for setting a rotationalspeed of the metering motor. These signals are, for example, suppliedfrom a not-shown host controller.

A deviation e1 between the backward position command pattern signal xij0and a screw position signal xijm is taken by an adder 32 using the screwposition signal xijm as a feedback signal. The injection motor 16 isfeedback-controlled based on the deviation e1.

The screw position signal xijm can be obtained based on the rotationaldisplacement of the injection motor 16 from a reference position.

A PID controller 33 for the backward position command calculates anoperation quantity u1 with which the screw position should be operated,based on the deviation e1. A speed calculator 34 calculates a speedcommand v1 based on the operation quantity u1. An adder 35 adds thebackward speed command pattern signal vij0 as a feed-forward signal tothe speed command v1 to obtain a backward speed control value v3. Aminimum value selector 36 selects a smaller one of a back-pressure speedcommand calculated value v2, which will be described later, and thebackward speed control value v3. The minimum value selector 36 outputsthe selected value as a screw backward speed command vij.

A servo amplifier 38 controls the rotation of the injection motor 16 inaccordance with the speed command vij. The rotational position of theinjection motor is supplied to the adder 32 through the servo amplifier38.

A PID controller 44 for setting the back pressure calculates anoperation quantity u2 based on a deviation e2 between the back pressurebp0 indicated by the backpressure setting pattern and the back pressuremeasured by the load cell 49, which deviation e2 is obtained by an adder43. A speed calculator 45 calculates a back-pressure speed command v2based on the operation quantity u2, and supplies the calculation resultto the minimum value selector 36. Thus, even when the back-pressurespeed command value v2 is excessively large to pass over the backwardposition of the screw 6, the screw 6 can be prevented from passing overthe position set by the backward position command pattern xij0

The metering motor rotational speed setting pattern signal vcg0 issupplied to a servo amplifier 47. The servo amplifier 47 controlsdriving of the metering motor 9 in accordance with the metering motorrotational speed setting pattern signal vcg0.

Patent Document 1: JP-A-5-345337

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

In the in-line screw type injection molding machine in the backgroundart, for example, the metering motor is coupled with the base coupledwith the rear end of the screw, and the injection motor rotationallydrives the screw shaft abutting against the base to axially move thebase along the screw shaft.

When an injection mechanism and a metering mechanism are configured thusto abut against each other, the configuration of the injection mechanismand the metering mechanism becomes complicated. In addition, when theinjection mechanism and the metering mechanism are designed to bescrewed to each other through the screw shaft and a nut body screwed onthe screw shaft, the screw will move forward or backward if theinjection motor is suspended during metering.

An injection molding machine using electric motors of such mechanismsincludes an injection motor for moving a screw forward/backward and ametering motor for rotating the screw. Cooperative control of the twoelectric motors is needed for molding. Accordingly, the control becomesso complicated and troublesome that it is difficult to obtain a desiredinjection rate and a desired pressure. Particularly during plasticizingin which the screw is moved backward due to the pressure of the resinwhile being rotated, the screw passes over its backward position easily.

The present invention was accomplished in consideration of theseproblems. An object of the invention is to provide an in-line screw typeinjection molding machine having a simplified configuration of theinjection mechanism, the metering mechanism and the control circuit.

Means for Solving the Problems

In order to attain the aforementioned problems, the present inventionuses the following means.

In an in-line screw type injection molding machine, a screw in a heatingcylinder is rotated to knead and plasticize raw resin while transferringthe resin toward a forward end side of the screw to thereby reservemetered molten resin on the front end side, and the screw is movedforward to inject the molten resin into a mold to thereby fill the moldwith the molten resin. The in-line screw type injection molding machineincludes: a metering motor which is disposed on a rear end side of thescrew and rotationally drives the rear end side of the screw; a screwmechanism which includes a nut body attached to the rear end side of thescrew and rotating integrally with the screw, and a screw shaft fittedto the nut body, the screw mechanism converting rotational motion of aninjection motor for rotationally driving the screw shaft into linearmotion of the screw through the nut body; an injection motor drivecircuit which drives the injection motor to allow a position of thescrew to follow a predetermined position command pattern; a meteringmotor drive circuit which drives the metering motor to allow rotation ofthe metering motor to follow a predetermined speed setting pattern; andan adder-subtracter circuit which adds or subtracts a speed settingpattern signal of the metering motor drive circuit to or from a speedcommand signal of the injection motor drive circuit so as to compensatean axial displacement of the screw caused by the rotation of themetering motor.

EFFECT OF THE INVENTION

Since the present invention has the aforementioned configuration, it ispossible to simplify the configuration of the injection mechanism, themetering mechanism and the control circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

The best embodiment will be described below with reference to theaccompanying drawings. FIG. 1 is a diagram for explaining the outline ofan in-line screw type injection molding machine according to theembodiment. In FIG. 1( a), the reference numeral 1 represents a headstock disposed on a not-shown injection unit base plate; 2, a retentionplate disposed likewise on the not-shown injection unit base plate so asto face the head stock 1 at a predetermined distance therefrom; 3, aheating cylinder whose rear end portion is fixed to the head stock 1; 4,a nozzle attached to the front end of the heating cylinder 3; 5, a bandheater wound on the outer circumference of the heating cylinder 3; 6, ascrew disposed rotatably and movably forward/backward in the heatingcylinder 3; and 1 a and 3 a, raw resin supply holes provided in the headstock 1 and the heating cylinder 3 respectively so that raw resindropping down and supplied from a not-shown hopper can be supplied intothe rear end portion of the heating cylinder 3.

In addition, the reference numeral 7 represents a connection bar whichis laid between the head stock 1 and the retention plate 2; 8, a linearmotion body provided on a not-shown rail member through a linear motionguide movably forward/backward between the head stock 1 and theretention plate 2; 9, an internally hollow built-in type metering motor(hereinafter referred to as “metering motor 9”) mounted on the linearmotion body 8; 10, a casing of the metering motor 9; 11, a cylindricalstator of the metering motor 9 which stator is fixed to the casing 10;12, a cylindrical rotor of the metering motor 9 which rotor can rotateinside the stator 11; 13, a sleeve which is fixed to the innercircumferential surface of the rotor 12 by tight fitting or the like;14, a bearing which is inserted between the casing 10 and the sleeve 13so as to rotatably support the sleeve 13; and 15, a rotary coupler whichis fixed to the sleeve 13 so as to fix the base end portion of the screw6.

In addition, the reference numeral 16 represents an internally hollowbuilt-in type injection motor (hereinafter referred to as “injectionmotor 16”) which is mounted on the retention plate 2; 17, a casing ofthe injection motor 16; 18, a cylindrical stator of the injection motor16 which stator is fixed to the casing 17; 19, a cylindrical rotor ofthe injection motor 16 which rotor can rotate inside the stator 18; and20, a sleeve which is fixed to the inner circumferential surface of therotor 19 by tight fitting or the like. Though shown by simplifiedillustration, the sleeve 20 is rotatably retained in the casing 17 witha not-shown bearing.

In addition, the reference numeral 21 represents a ball screw mechanismfor converting the rotation of the injection motor 16 into linearmotion; 22, a screw shaft of the ball screw mechanism 21 (rotary portionof the ball screw mechanism 21) which is rotatably retained on theretention plate 2 through a bearing 24; 23, a nut body of the ball screwmechanism 21 (linear motion portion of the ball screw mechanism 21)which is screwed on the screw shaft 22 so as to linearly move along thescrew shaft 22 in accordance with the rotation of the screw shaft 22while an end portion of the nut body 23 is fixed to the sleeve 13 of themetering motor 9 side directly or through a suitable member; and 25, acoupler which couples and fixes the sleeve 20 of the injection motor 16side and an end portion of the screw shaft 22 to each other.

In this embodiment, in a metering process, the metering motor 9 isdriven and controlled by rotational speed (rotational frequency)feedback control in accordance with a command from a control circuitthrough a servo amplifier which will be described later. The controlcircuit which will be described later administers the control of themachine (injection molding machine) as a whole. As a result, the screw 6rotates in a predetermined direction integrally with the sleeve 13 andthe rotary coupler 15. In a typical metering operation, due to therotation of the screw 6, raw resin supplied from a not-shown hopper tothe rear end side of the screw 6 through the raw resin supply holes 1 aand 3 a is kneaded and plasticized while being moved forward by thescrew feed action of the screw 6. According to this embodiment, however,the nut body 23 fixed to the sleeve 13 also rotates when the screw 6rotates in the predetermined direction. Thus, due to the rotation of thenut body 23 caused by the rotational driving of the screw 6, the nutbody 23 moves linearly along the screw shaft 22. Therefore, in order tocancel the linear motion of the nut body 23 (linear motion of themetering motor 9 or the screw 6) due to the rotation of the nut body 23caused by the rotational driving of the screw 6, the control circuitdrives and controls the injection motor 16 through a servo driver 38which will be described later. The control is performed by pressurefeedback control with a set back pressure as an intended value. Thus,the screw 6 is moved backward by proper control as the molten resin isfed to the front end side of the screw 6 while the back pressure imposedon the screw 6 is kept in a predetermined pressure. For example, whenthe metering motor 9 is rotated at 10 revolutions per unit time, theinjection motor 16 is rotated at 9.9 revolutions per unit time. Thus,the screw 6 is controlled to be given a predetermined back pressurewhile the linear motion of the nut body 23 due to the rotation of thenut body 23 caused by the rotational driving of the screw 6 iscancelled. As soon as one shot of molten resin is reversed on the frontend side of the screw 6, the rotational driving of the screw 6 by themetering motor 9 is suspended.

On the other hand, in an injection/filling process, at a proper timingafter the completion of metering, the injection motor 16 is driven andcontrolled by speed feedback control with the servo driver 38 which willbe described later, in accordance with a command from the controlcircuit which will be described later. As a result, the rotation of theinjection motor 16 is converted into linear motion by the ball screwmechanism 21, and the linear motion is transmitted to the screw 6through the aforementioned linear motion transmission system. Thus, thescrew 6 is driven to move forward suddenly, so that the molten resinreserved on the front end side of the screw 6 can be injected into acavity of a not-shown mold which has been clamped, and the cavity can betherefore filled with the resin. Thus, a primary injection process isachieved. In a pressure holding process following the primary injectionprocess, the injection motor 16 is driven and controlled by not-shownpressure feedback control through the servo driver 38 in accordance witha command from the control circuit. Thus, a holding pressure which hasbeen set is given from the screw 6 to the resin in the not-shown mold.

FIG. 1( b) is a diagram for explaining the driving directions of themetering motor 9 and the injection motor 16 during injection of theresin, during metering and during back injection (suck back).

As shown in FIG. 1( b), (1) during injection, only the injection motor16 is rotated backward (clockwise in view from the motor mountingplane). Thus, the screw 6 is driven to the left suddenly. (2) Duringmetering (also referred to as “during plasticizing”, when raw resin isplasticized and metered), the metering motor 9 is rotated forward(counterclockwise in view from the motor mounting plane). Thus, theresin kneaded and plasticized in the heating cylinder is moved towardthe front end side (left side) of the screw, and metered molten resin isaccumulated on the front end side.

As mentioned above, the nut body 23 coupled with the screw 6 alsorotates forward (counterclockwise in view from the direction of theillustrated arrow) when the screw 6 is rotationally driven forward(counterclockwise in view from the direction of the illustrated arrow)by the metering motor 9. If the injection motor 16 for rotationallydriving the screw shaft 22 screwed down to the nut body 23 were stillsuspended on this occasion, the nut body 23 and the screw 6 coupledtherewith would move to the illustrated right.

In order to cancel the movement, the injection motor 16 is rotationallydriven in the same (backward) direction as the metering motor 9 by thesame displacement as described previously. When the rotationaldisplacement of the injection motor 16 is set to be slightly smallerthan that of the metering motor 9, the screw 6 can be moved to the rightgradually to give an optimal back pressure to the screw 6. (3) Duringback injection, the injection motor 16 is driven forward to drive thescrew 6 to the illustrated right in the state where driving the meteringmotor 9 has been suspended. Thus, leakage of resin from the nozzle 4 canbe suppressed.

FIG. 2 is a diagram for explaining the control circuit for controllingdriving of the injection motor 16 and the metering motor 9. In FIG. 2,xij0 designates a backward position command pattern signal indicatingthe backward position of the screw; vij0, a backward speed commandpattern signal indicating the backward speed of the screw; bp0, a backpressure setting pattern signal for setting a back pressure imposed onthe screw; and vcg0, a metering motor rotational speed setting patternsignal for setting a rotational speed of the metering motor. Thesesignals are, for example, supplied from a not-shown host controller.

A deviation e1 between the backward position command pattern signal xij0and a screw position signal xijm calculated in an adder 52 as will bedescribed later is taken by an adder 32 using a screw position signalxijm as a feedback signal. The injection motor 16 is feedback-controlledbased on the deviation e1.

The screw position signal xijm can be obtained from the differencebetween the rotational displacement (the absolute value of therotational angle) of the injection motor 16 from a reference positionand the rotational displacement (the absolute value of the rotationalangle) of the metering motor 9 from a reference position (when therotational directions of the two motors coincide with each other) or thesum of those rotational displacements (when the rotational directions ofthe two motors do not coincide with each other). The calculation of thesum or the difference can be performed by changing a coefficient of amultiplier 51.

A PID calculator 33 for the backward position command calculates anoperation quantity u1 with which the screw position should be operated,based on the deviation e1. A speed calculator 34 calculates a speedcommand v1 based on the operation quantity u1. An adder 35 adds thebackward speed command pattern signal vij0 as a feed-forward signal vffto the speed command v1 to obtain a backward speed control value v3. Aminimum value selector 36 selects a smaller one of a back-pressure speedcommand calculated value v2, which will be described later, and thebackward speed control value v3. The minimum value selector 36 outputsthe selected value as a screw backward speed command value v4.

An adder 37 adds (when the rotational directions of the two motors donot coincide with each other) or subtracts (when the rotationaldirections of the two motors coincide with each other) the meteringmotor rotational speed setting pattern signal vcg0 (rotation correctionvalue v5) to or from the screw backward speed command value v4, andsupplies the calculation result as a speed command vij to the servoamplifier 38. The servo amplifier 38 controls the rotation of theinjection motor 16 in accordance with the speed command vij. Therotational position of the injection motor 16 is measured by an encoderattached to the injection motor 16, and supplied to the adder 52 throughthe servo amplifier 38.

A PID controller 44 for setting the back pressure calculates anoperation quantity u2 based on a deviation e2 between the back pressurebp0 indicated by the back pressure setting pattern and the back pressuremeasured by the load cell 49, which deviation e2 is obtained by an adder43. A speed calculator 45 calculates a back-pressure speed command v2based on the operation quantity u2, and supplies the calculation resultto the minimum value selector 36. Thus, even when the back-pressurespeed command value v2 is excessively large to pass over the backwardposition of the screw 6, the screw 6 can be prevented from passing overthe position set by a backward position command xij.

The metering motor rotational speed setting pattern signal vcg0 issupplied to a servo amplifier 47. The servo amplifier 47 controlsdriving of the metering motor 9 in accordance with the metering motorrotational speed setting pattern signal vcg0.

As mentioned previously, the metering motor 9 rotational speed settingpattern signal vcg0 supplies the position where the screw 6 should bemoved backward due to the rotation of the metering motor, as a rotationcorrection signal v5 to the adder 37. That is, the rotational speedsetting pattern signal vcg0 (rotation correction value v5) of themetering motor 9 is added to (when the rotational directions of the twomotors do not coincide with each other) or subtracted from (when therotational directions of the two motors coincide with each other) therotational speed command value (screw backward speed command value v4)of the injection motor 16 so as to correct the speed command vij for theinjection motor servo amplifier 38 in advance. Thus, the speed commandvij can be outputted to the injection motor servo amplifier 38 quicklyin accordance with a change of the position where the screw 6 should bemoved backward due to the rotation of the metering motor 9.

FIG. 3 is a diagram for explaining changes of various control quantitiesin the control circuit shown in FIG. 2. In FIG. 3, (a) designates thescrew position xij0; (b), the screw backward speed control value v3 andthe screw backward speed command value v4; (c), the injection speedcommand vij and the metering motor rotational speed setting patternsignal (value corrected in the rotation direction) v5; (d), the backpressure setting pattern signal bp0; and (e), the metering motorrotational speed command vcg0.

As shown in FIG. 3( b), of the screw backward speed control value v3calculated from the backward position command pattern xij0 and thebackward speed command pattern vij0 and the back-pressure speed commandcalculated value v2 calculated from the back pressure setting patternbp0, a smaller one is selected as the screw backward speed command v4.

As shown in FIG. 3( c), the rotation-direction correction value v5 ofthe metering motor rotational speed setting pattern signal is added tothe screw backward speed command v4 to generate the speed command vijfor the injection motor servo amplifier 38.

As described above, according to this embodiment, as shown in FIG. 1, ametering mechanism and an injection mechanism using an in-line screw canbe formed integrally. It is therefore possible to form the meteringmechanism and the injection mechanism compactly and inexpensively. Inaddition, when the injection mechanism and the metering mechanism areformed integrally, operational interference may occur between thesemechanisms. However, the interference can be canceled by adding changes(for example, the adders 52 and 37 shown in FIG. 2) to the controlcircuit. That is, according to this embodiment, the injection mechanismand the metering mechanism can be integrated to reduce the cost, whilethe complication of control caused by the integration can be absorbed byadding the changes to the control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram for explaining the outline of an in-line screw typeinjection molding machine according to an embodiment.

FIG. 2 A diagram for explaining a control circuit for controllingdriving of an injection motor and a metering motor.

FIG. 3 A diagram for explaining changes of various control quantities inthe control circuit shown in FIG. 2.

FIG. 4 A diagram for explaining a control circuit of an in-line screwtype injection molding machine in the background art.

DESCRIPTION OF REFERENCE NUMERALS

-   1 head stock-   1 a raw resin supply hole-   2 retention plate-   3 heating cylinder-   3 a raw resin supply hole-   4 nozzle-   5 band heater-   6 screw-   7 connection bar-   8 linear motion body-   9 metering built-in type motor (metering motor)-   10 casing-   11 stator-   12 rotor-   13 sleeve-   14 bearing-   15 rotary coupler-   16 injection built-in type motor (injection motor)-   17 casing-   18 stator-   19 rotor-   20 sleeve-   21 ball screw mechanism-   22 screw shaft (rotary portion of ball screw mechanism)-   23 nut body (linear motion portion of ball screw mechanism)-   24 bearing-   25 coupler-   32 adder-   33 PID calculator-   34 speed calculator-   35 adder-   36 minimum value selector-   37 adder-   38 servo amplifier-   43 adder-   44 PID calculator-   44 speed calculator-   47 servo amplifier-   49 load cell-   50,51 multiplier-   52 adder-   57,58 encoder

1. An in-line screw type injection molding machine in which a screw in aheating cylinder is rotated to knead and plasticize raw resin whiletransferring the resin toward a forward end side of the screw to therebyreserve metered molten resin on the front end side, and the screw ismoved forward to inject the molten resin into a mold to thereby fill themold with the molten resin, the in-line screw type injection moldingmachine being characterized by comprising: a metering motor which isdisposed on a rear end side of the screw and rotationally drives therear end side of the screw; a screw mechanism which includes a nut bodyattached to the rear end side of the screw and rotating integrally withthe screw, and a screw shaft fitted to the nut body, the screw mechanismconverting rotational motion of the screw shaft into linear motion ofthe screw through the nut body; an injection motor which rotationallydrives the screw shaft as a constituent member of the screw mechanism;an injection motor drive circuit which drives the injection motor toallow a position of the screw to follow a predetermined position commandpattern; a metering motor drive circuit which drives the metering motorto allow rotation of the metering motor to follow a predetermined speedsetting pattern; and an adder-subtracter circuit which adds or subtractsa speed setting pattern signal of the metering motor drive circuit to orfrom a speed command signal of the injection motor drive circuit so asto compensate an axial displacement of the screw caused by the rotationof the metering motor.
 2. An in-line screw type injection moldingmachine according to claim 1, characterized by further comprising: aninjection motor rotation encoder which measures a rotational position ofthe screw shaft, and a metering motor rotation encoder which measures arotational position of the metering motor, wherein an output of themetering motor rotation encoder is added to or subtracted from an outputof the injection motor rotation encoder to measure the position of thescrew.
 3. An in-line screw type injection molding machine according toclaim 2, characterized in that: the injection motor rotation encoder andthe metering motor rotation encoder are absolute type encoders eachoutputting an absolute value of a rotational position.
 4. A method forcontrolling an in-line screw type injection molding machine in which ascrew in a heating cylinder is rotated to knead and plasticize raw resinwhile transferring the resin toward a forward end side of the screw tothereby reserve metered molten resin on the front end side, and thescrew is moved forward to inject the molten resin into a mold to therebyfill the mold with the molten resin, the in-line screw type injectionmolding machine including a metering motor which is disposed on a rearend side of the screw and rotationally drives the rear end side of thescrew, and a screw shaft which is fitted to a nut body attached to therear end side of the screw and rotating integrally with the screw,rotational motion of an injection motor being converted into linearmotion of the screw through the screw shaft, the method beingcharacterized by comprising the steps of: driving the injection motor toallow a position of the screw to follow a predetermined position commandpattern, and driving the metering motor to allow rotation of themetering motor to follow a predetermined speed setting pattern; andadding or subtracting a speed setting pattern signal of the meteringmotor drive circuit to or from a speed command signal of the injectionmotor drive circuit so as to compensate an axial displacement of thescrew caused by the rotation of the metering motor.